阿尔金活断层的古地震与分段

活断层的破裂过程往往是通过多个独立破裂段落的组合而进行的。活断层的分段就是对断层上各个稳定的独立破裂单元的识别。这是深入认识断层的活动习性特征和进行潜在地震危险性评估的重要途径。古地震资料是识别破裂单元最直接的依据。本文在根据障碍构造对阿尔金活断层进行几何结构分段的基础上，结合古地震资料的分析进行了破裂分段的识别，划分了１１个破裂段。在约２００００年来各个段落活动时段、强度、古地震复发间隔都有所不同。总趋势是由西向东逐渐减弱。在约４５００—２０００ａＢ．Ｐ．存在一个各个段落都比较活跃的阶段。这与沿青藏高原边缘的一些活动带上揭露出的情况很相似，看来这是与整个高原的构造活动在这个时段曾有所加强有关。

PALEOEARTHQUAKES ALONG THE ALTUN ACTIVE FAULT AND ITS SEGMENTATION

The Altun active fault is a large strike-slip fault with manifest dislocations.The newly activities of the fault are mainly along a principal sliding plane. This principal sliding plane extends from Xorkol on west, through Kuyak, Mandalike,Qingshuiquan, Akatutage, Horkor, Annanba, Aksay and Subei eastward to Kuantanshan, the whole length being about 1600km. Along this principal sliding plane, a series of surface ruptures, deformations and paleoearthquake traces can be found.The existence of numerous paleoearthquake traces proved that the Altun fault has been a very active fault in Holocene. The most active segments were KuyakAqiang, Tula-Huangtuquan, and Uzunxiaol-Aksay.Many clear paleoseismic deformation belts are preserved along the Altun fault.The so called paleoseismic deformation belt refers to the areas where residual deformations caused by one or more paleoseismic events are concentrated and preserved. Twelve paleoseismic belts have been identified, and they are further divided into 28 sub-segments in terms of differences of deformation and other paleoseismic traces.As a whole, the Altun active fault is quite continuous with a flat wave-shaped trace. she secondary faults are obviously of en echelon arrangement, predominantlyin the form of right-step. The segmentation of the Altun fault is of hierarchy, i.e.,a large segment can be divided into subsegments at several levels. Three-level segmentation is suitable for the fault.Segments at the largest level are determined on the basis of active tectonicunits and fault activity. The entire Altun fault is made up of four large segment&;arranging in the form of right-step. Among the four large segments there existthree large barriers: the Tula dual stepover, the Akatuta stepover and the Yulekenstepover. The distribution of paleoseismic ruptures also indicates the existence ofthe four segments because paleoearthquakes at different segments took place in different time intervals.Segmentation .at the second level is based on the rupture coverage of the largest earthquake. The Altun fault is located in the highly compressional belt betweenthe Tibet Plateau and the Tarim Basin, along which there are a lot of lensing com-pressional structural blocks. New fault ruptures have been developed in the arc margins of these compressional blocks. The Altun active fault was just formed by linking of these arc ruptures, and so show a wavelike extending as a whole. Now.at least eleven arc structural segments of the kind can be identified. They are varying in length and arrange in the form of right step, spaced out by some stopovers or interruptions. The convex parts of the arc structural segments point to theNEE direction because of the northward compression of the Tibet Plateau. The segmentation of the Altun fault is controlled by the arc structural segments: an arcstructural is right a persistent rupture segment.Each arc segment is often composed of several en echelon arranged secondaryfaults, which constitute the base of the segmentation at the third level. Froal theobtained data, we can judge that the number of paleoseismic events (faulting events>on the different segments (or different profiles) along the Altun fault since LateEpipleistocene has been varying, generally from 2--3 times to more than 5 timesAll of these events are large earthquakes of more than M7 by the estimate of scaleand displacement of residual deformation belts.The temporal and spatial distribution of these paleoseismic events reflect. certain segmentation characteristic along the whole fault. Four big segments can bedivided, of which the west two ones extends from Xorkol to Aqiang and the faulting time was mainly between 1 000 a B.P. and 4 400 aB.P.; the central one extendsfrom Tula to Yuleken and the faulting time was between 1 500 aB. P. and 7 000a B.P.; and the east one extends from Aksay to Bagexia and the faulting time wasbetween 7 000 a B.P. and 18 000 a B.P. The average recurrence interval for earthquakes on all segments is 4 400--l 1 000 a But, if events are grouped by stage.the recurrence interval calc